Data storage devices such as the disk drive shown in FIG. 1 comprise a disk 2 and a head 4 actuated radially over the disk 2. The disk 2 comprises a plurality of radially spaced, concentric tracks 6 for recording user data sectors and servo sectors 81-8N. The servo sectors 81-8N comprise head positioning information (e.g., a track address and servo bursts) read by the head 4 and processed by a servo control system in control circuitry 10 to seek the head 4 from track to track as well as track the centerline of a data track during access operations (write/read operations).
As the radial density of the data tracks increases to achieve higher capacity, disk drives may employ multiple actuators for actuating the head 4 over the disk 2. For example, the prior art disk drive shown in FIG. 1 utilizes three actuators including a primary actuator in the form of a voice coil motor (VCM) 12 configured to rotate an actuator arm 14 about a pivot, a first secondary actuator 16 configured to actuate a suspension 18 relative to the actuator arm 14, and a second secondary actuator 20 configured to actuate a distal end of the suspension 18 near the head slider. This configuration is referred to as a triple stage actuator with the second and third stage having more error rejection capability at lower frequency range (higher slope in the sensitivity function) as the actuators get closer to the head element (due to moving a smaller mass). Although the triple stage actuator improves the servo performance by improving the sensitivity function of the servo loop, each additional secondary actuator increases the cost and manufacturing complexity of the disk drive.